I would like to style the Calendar inside Thunderbird (115.11.1 on MacOS) without using any extensions/add-ons. Specifically, I would like to modify the looks of recurring events. I enabled userChrome.css (toolkit.legacyUserProfileCustomizations.stylesheets) and the “has:” selector (layout.css.has-selector.enabled). In my userChrome.css, I added styles:

li:has([alt*="Recurring"]) { background-color:grey !important; padding:1rem !important; }
li:has(*[alt*="Recurring"]) { background-color:yellow !important; padding:1rem !important; }

Unfortunately, this seems to have no effect upon restarting Thunderbird. However, when I open Developer Toolbox (Tools > Developer Toolbox > Developer Toolbox > Inspector tab) suddenly everything works and the stylesheets are applied, but only until Thunderbird is shut down. Other custome styles in Calendar seem to work ok.

Is this intended behavior or a bug? Can you let me know if it replicates with your Thunderbird?

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